Many commercially significant applications exist for methods by which the electrical properties of rocks may be measured. For example, the importance of high frequency dielectric constant measurements in the field of petroleum exploration and production is evidenced by the recent emergence of frequency-domain dielectric logging tools, operating at about 20 MHz and at 1.1 GHz. Measurement of the complex dielectric constant (also referred to as "complex dielectric permittivity") and conductivity of rocks can be used to evaluate important reservoir properties such as porosity, oil/gas saturation, and mineral composition. Dielectric measurements at high frequency, particularly in the MHz through GHz range, are especially useful because they can provide reservoir information that is less affected by brine salinity than that obtainable using low frequency devices such as induction logging tools.
Throughout this specification, including the claims, the term "rock" will be used to denote the broad class of mineral masses or aggregates including those rocks associated with a rigid matrix, clays associated with a semi-rigid matrix, and porous rocks saturated by any liquid, gas, or liquid gas mixture.
Frequency domain techniques have been developed for measuring the dielectric properties of rocks. For example, a borehole logging tool, capable of measuring the travel time and attenuation of an electromagnetic wave having frequency 1.1 GHz between two receivers disposed in a borehole, is described in Wharton, et al., "Electromagnetic Propagation Logging: Advances in Technique and Interpretation," Paper SPE 9267 presented at the SPE Annual Technical Conference and Exhibition, Dallas, Tex., on Sept. 21-24, 1980. Wharton, et al. discloses determining the dielectric constant of a subterranean earth formation adjacent to the borehole by analysis of the measured travel time and attenuation. A laboratory system for determination of complex dielectric permittivity and conductivity is described in Rau, et al., "Measurement of Core Electrical Parameters at Ultrahigh and Microwave Frequencies", Journal of Petroleum Technology, November 1982, pp 2689-2700. Rau, et al. discloses the technique of reflecting from (or transmitting through) a machined rock sample an electromagnetic wave having a selected frequency in the range 100 MHz to 2 GHz. The sample holder disclosed in Rau, et al. is a rigid, coaxial, air-filled transmission line with standard coaxial connectors at each end. The measured rock sample must be machined to fit tightly in the space between the center and outer conductors of the coaxial transmission line, and the sample must be cut to a known precise length. G. S. Huchital, et al., "The Deep Propagation Tool (A New Electromagnetic Logging Tool)", Paper SPE 10988, presented at the 56th Annual Fall Tech. Conference, 1981, discloses an electromagnetic logging tool operating at a frequency in the tens of MHz range. The Huchital, et al. tool measures phase shift and attenuation of an electromagnetic wave propagating between receivers disposed in a borehole.
J. P. Poley, et al., "Use of V.H.F. Dielectric Measurements for Borehole Formation Analysis", The Log Analyst, 1978 (May-June), pp. 8-30; and R. P. Mazzagatti, et al., "Laboratory Measurement of Dielectric Constant Near 20 MHz", presented at the SPE 58th Annual Technical Conference and Exhibition, San Franciso, Calif., on Oct. 5-8, 1983, also disclose frequency-domain techniques for measuring rock dielectric properties. Poley, et al. discloses techniques for making measurements at selected frequencies in the 1.5 KHz through 500 MHz range and in the 300 MHz through 2.4 GHz range. For measurements in the 1.5 KHz through 500 MHz range, Poley, et al. discloses measuring disk shaped rock samples placed between the parallel plate electrodes of a sample holder. For measurements in the 300 MHz to 2.4 GHz range, Poley, et al. discloses measuring machined samples disposed in the annular region between the conductors of a coaxial transmission line. Mazzagatti, et al. discloses measuring cylindrical rock samples held between the parallel plates of a cell holder by determining the reflection coefficient of an electromagnetic wave (having selected frequency from the range 2-100 MHz) as the electromagnetic wave is caused to reflect from the rock sample.
Frequency-domain techniques of the types referenced above permit determination of the complex dielectric constant at only one frequency as the result of each measurement. To measure conductivity, and to extract dielectric constant information at a broad range of frequencies, conventional frequency domain techniques require time consuming multiple measurements at each of a number of different frequencies. In the case of conventional dielectric logging tools of the type referenced above, it is particularly difficult and time consuming to make measurements at several different frequencies because each such measurement requires use of a different tool. Conventional laboratory techniques of the type referenced above additionally require the difficult and time consuming step of machining solid samples to fit closely into sample cells followed by data collection at a variety of frequencies.
The method of the present invention is not a frequency domain technique. It is instead, a time-domain dielectric spectroscopic technique by which the frequency dependence of a rock's complex dielectric constant over a broad frequency range of from about 1 MHz to several GHz, as well as the rock's conductivity, may be determined in a single measurement. Throughout this specification, time-domain spectroscopy will sometimes be denoted as "TDS". TDS facilitates determination of a sample's electrical properties from real time measurements of transient currents which follow application of a voltage pulse to the sample. The theory of TDS is discussed in Fourier, Hadamard, and Hilbert Transforms in Chemistry, edited by A. G. Marshall, pp 183-206 (Plenum Press, New York and London 1982).
TDS techniques have been applied to determine electrical properties of liquids. See for example, U. Kaatze, et al., "Dielectric Relaxation Spectroscopy of Liquids: Frequency Domain and Time Domain Experimental Methods", J. Phys. E: Sci. Instrum., 13, 1980, pp 133-134; and M. J. C. van Gemert, "High-Frquency Time-Domain Methods in Dielectric Spectroscopy", Phillips Res. Repts., 28, 1973, pp 530-572. Similarly, TDS techniques have been applied to measure electrical properties of powders pressed into a coaxial line sample cell. See B. C. Bunker, et al., "A Study of the Rate of Intervalence Electron Transfer Using Time Domain Reflectometry", J. Am. Chem. Soc., 103, 1981, pp 4254-4255; and B. C. Bunker, et al., "Electron-Transfer Rates in Mixed-Valence Europium Sulfide by Time Domain Reflectrometry", J. Am. Chem. Soc., 104, 1982, pp 4593-4598.
Use of a TDS technique for measuring dielectric properties of oil shale has also been disclosed in M. F. Iskander, "A Time-Domain Technique for Measurement of the Dielectric Properties of Oil Shale During Processing", Proceedings of the IEEE, 69, No. 6, June 1981, pp 760-762. The Iskander, et al. paper discloses use of a small shunt capacitor terminating a coaxial line section as a sample holder. The sample to be measured is positioned to fill a gap between the inner conductor of the coaxial line and a terminating metal plate. Thus, the size of samples that can be measured in the Iskander, et al. system is limited by the size of the gap between the inner conductor and terminating metal plate of the sample holder. It would thus be impractical to use the Iskander, et al. system for measuring the properties of rock samples (or rock formations) that are too large to be accommodated in the sample holder of such system.